Ultrasonics is a branch of acoustics dealing with vibratory waves at frequencies above the average human hearing range, i.e., frequencies over 20 kHz. In contrast, sound waves with frequencies in the range of 20 Hz to 20 kHz are in the audible range, whereas sound waves with frequencies below 20 Hz are in the infrasonic range. An ultrasonic wave is longitudinal, travels as concentric hollow spheres and causes a series of compressions and expansions of the molecules in the medium surrounding it as it propagates. It can be used in various engineering applications.
Ultrasonics is a trade term coined by the Ultrasonic Manufacturers Association and used by its successor, the Ultrasonic Industry Association, to refer to the use of high-intensity acoustic energy to change materials. This usage is contrasted to ultrasound, which is generally reserved for imaging, as in sonar, materials examination, i.e. non destructive inspection (NDI), and diagnostics (mammography, doppler bloodflow, etc.). However, in spite of this distinction, much technical material on ultrasound imaging actually uses the term ultrasonics.
Ultrasonication offers great potential in the processing of liquids and slurries, by improving the mixing and chemical reactions in various applications and industries. Ultrasonication generates alternating low-pressure and high-pressure waves in liquids, leading to the formation and violent collapse of small vacuum bubbles. This phenomenon is termed cavitation and causes high speed impinging liquid jets and strong hydrodynamic shear-forces. These effects are used for the deagglomeration and milling of micrometer and nanometer-size materials as well as for the disintegration of cells or the mixing of reactants. In this aspect, ultrasonication is an alternative to high-speed mixers and agitator bead mills. Ultrasonic foils under the moving wire in a paper machine will use the shock waves from the imploding bubbles to distribute the cellulose fibres in a more uniform manner in the produced paper web, which will thus culminate in the making of a stronger paper with a more even surface profile. Furthermore, chemical reactions benefit from the free radicals created by the cavitations as well as from the energy input and the material transfer through boundary layers. For many processes, this sonochemical effect leads to a substantial reduction of the reaction time, like in the transesterification of oil into biodiesel. Ultrasonication can easily be tested in lab scale for its effect on various liquid formulations. Equipment manufacturers have developed a number of larger ultrasonic processors of up to 16 kW power. Therefore volumes from 1 mL up to several hundred gallons per minute can be sonicated today in order to achieve all kinds of results.
The low-intensity ultrasonic waves, typically <1 W cm−2 are non-destructive where it will never change the physical or chemical state of the medium due to its small power level. This non-invasive technology has been applied in quality assessment and provides information about physicochemical properties, such as composition, structure, physical state and flow-rate.
The application of high intensity ultrasonic waves, typically in the range 10-1000 Wcm−2 can cause physical disruption of a material or promote certain chemical reactions. High intensity ultrasound has been used in various applications ranging from cell disruption, modification and control of crystallization processes, enzyme deactivation, meat tenderization, enhancement of oxidation and ultrasonic mixing.
In general, ultrasonic technology has been widely used for mixing purposes in the industry.
US20090168591 discloses an ultrasonic mixing system for mixing particulate including rheology modifiers, sensory enhancers, pigments, lakes, dyes, abrasives, absorbents, anti-caking, anti-acne, anti-dandruff, anti-perspirant, binders, bulking agents, colorants, deodorants, exfoliants, opacifying agents, oral care agents, skin protectants, slip modifiers, suspending agents, warming agents and combinations thereof into formulation in a treatment chamber.
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U.S. Pat. No. 5,059,309 teaches a continuous ultrasonic flotation unit which permits a mixture to be ultrasonically agitated as it is passed through a small mixing chamber.
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Mixing is a key step during the production of dough based products, which allows for the flour, water, and other ingredients if present, such as salt, chemical leavening agents, and/or yeast to be assimilated thereby forming a. coherent mass. It has been noted that air is also an important ingredient incorporated during mixing as it often goes unmentioned as an ingredient. The presence of air bubbles attribute to the taste sensation and the mouth-feel of food making it important in food assortments. The creation and control of aerated structures in cereal-based food such as bread, cakes and biscuits is the key to mastering the manufacture of these products as they gain their distinctive appeal from their aerated structure. Sonication is the commonly used method for micro-bubble generation besides mechanical agitation to manufacture aerated structures in these cereal based food products.